I don't know Toyota engines that well, but I know they have a lot of trouble with timing chain guides from what I have heard. Also if the rockers have a ball socket under the adjustment like Nissan then they could be worn and have a ridge that will make adjusting them impossible because it rides up on the ridge and gets tight or down on the worn part and is loose. I am sure there is a good answer for you, I just don't have one beeing I have only worked on a couple Toyota motors.